A large number of bolts and nuts are used for attaching various devices and components in the process of manufacture and assembly of automobiles and other vehicles. When a bolt is attached to a device, during the manufacturing process, tightening is carried out by selecting a nut that matches the bolt. When a nut is secured to a device or when a screw thread is formed in a device, a bolt is selected to perform the tightening operation. In addition, when mounting holes are formed in a device or frame, bolts are inserted into these holes and tightening is carried out by selecting nuts that match the bolts. If necessary, washers are used by slipping them onto bolts. In any case, tightening has to be performed by correctly selecting bolts or nuts according to design specifications and using tightening torques that are in compliance with design specifications.
In recent years, vehicle production lines or vehicle production equipment have come to be used for the production of vehicles that are not necessarily of the same type. Nowadays, vehicles of different types with slightly different specifications are handled together on a single assembly/production line. The well-known “Kanban system” is used in such cases. The articles being processed, which move along a production line, are provided with “Kanban” or signboards showing details of their manufacturing specifications. By looking at the signboards of the successively arriving processed articles, i.e. work objects, operators in each work area can get information about their specifications and select components, bolts, nuts, etc., or apply different tightening torques depending on their specifications, as well as make other fine adjustments. This permits organized and continuous manufacture of products with correct specifications in a flexible, responsive manner even if the specifications of the products are partially different.
A device, in which a torque sensor and other sensors are attached to an electric driver and the outputs of the sensors are fed to a personal computer device, is described in Patent document 1 below as an example of a conventional device used for bolt or nut tightening during product assembly work. It is understood that such a device provides an operator with instructions based on product specifications and, at the same time, controls the electric driver. According to the description, in such a device, the personal computer also controls other things such as the number of screws and their tightening torques.
Patent document 2 listed below discloses a device which, along with controlling an electric driver via a personal computer, has a torque sensor attached to the electric driver and controls the number of screws to be tightened and the number of items to be tightened. In addition, as mentioned in the document, the quality of assembly is improved by referring to a workflow procedure stored in the personal computer in advance.
Patent document 3 listed below discloses a device that detects power currents supplied to electric tools and controls the number of operations performed by the electric tools by measuring the frequency with which the value of the power currents exceeds a constant value.    Patent Document 1: JP H6-47634A (Japanese Patent No. 2765385)    Patent Document 2: JP H11-129164A    Patent Document 3: JP2003-123050A